(i) Field of the Invention
The present invention relates to an in-building CATV system connected to a lead-in wire from an external bi-directional CATV system for transmitting signals between the external bi-directional CATV system and an in-building terminal unit, an up-converter for converting the frequency of a signal inputted from the in-building terminal unit to an in-building upward signal in the in-building CATV system, and to a down-converter for converting the frequency of the in-building upward signal transmitted from the up-converter to the transmission frequency in the bi-directional CATV system to transmit the signal onto the transmission path of the bi-directional CATV system.
(ii) Description of the Related Art
In a conventional known in-building CATV system, a downward signal inputted via the lead-in wire (specifically, the downward signal transmitted from the central apparatus of the bi-directional CATV system) is transmitted to a terminal unit in a building via a transmission line in the building. Additionally, an upward signal inputted from the terminal unit on a subscriber side is transmitted, via the transmission line in the building, to the lead-in wire, and outputted toward the central apparatus of the bi-directional CATV system via the lead-in wire.
Moreover, in the in-building CATV system, noises generated in the building are superposed to the in-building transmission line, flow together, and are outputted to the external bi-directional CATV system together with the upward signal.
To reduce the confluent noises flowing out to the external bi-directional CATV system, in a heretofore proposed method, the upward signal generated in the subscriber terminal unit such as a cable modem (the signal with a frequency band, for example, of 5 to 42 MHz which can be transmitted by the bi-directional CATV system) is once frequency-converted in an up-converter to an upward signal (hereinafter referred to as the in-building upward signal) with a UHF band (e.g., 913 to 950 MHz) which is higher than the original frequency and which does not overlap the transmission frequency (e.g., 54 to 860 MHz) of the downward signal. The in-building upward signal is transmitted toward the lead-in wire, and the down-converter is used to convert the frequency of the in-building upward signal to the original frequency (in other words, the transmission frequency of the upward signal in the bi-directional CATV system) immediately before the upward signal from the in-building transmission line is outputted to the lead-in wire.
On the other hand, when the upward signal is frequency-converted to the in-building upward signal by the up-converter of the in-building CATV system, and the in-building upward signal is frequency-converted to the original upward signal by the down-converter of the in-building CATV system, the confluent noises outputted to the external bi-directional CATV system from the in-building CATV system can be reduced. However, when the up-converter and the down-converter are constituted to use reference signals from their respective oscillation circuits and perform the frequency conversion, a deviation is generated in the frequency of a frequency-converting reference signal obtained in each oscillation circuit, and in some cases the down-converter cannot convert the in-building upward signal to the original upward signal generated by the subscriber terminal unit
Wherefore, an object of the present invention is to provide an in-building CATV system in which the frequency of an upward signal is once converted to a high frequency by an up-converter on a terminal side and transmitted on a transmission line in a building, and the frequency of the in-building upward signal is returned to an original frequency by a down-converter before outputted to an external bi-directional CATV system, so that confluent noises outputted to the external bi-directional CATV system are reduced, and converting reference signals used for frequency conversion in both the converters are allowed to accurately coincide with each other with a simple constitution.
To attain this and other objects, according to one aspect of the present invention, there is provided an in-building CATV system comprising: a down-converter disposed between a lead-in wire and an in-building transmission line for outputting a downward signal inputted via the lead-in wire toward an in-building transmission line, using a pilot signal included in the downward signal to frequency-convert an in-building upward signal inputted via the transmission line to an upward signal corresponding to a transmission frequency in a bi-directional CATV system, and outputting the upward signal toward the lead-in wire; and an up-converter disposed between the in-building transmission line and a subscriber terminal unit for outputting the downward signal transmitted on the in-building transmission line toward the subscriber terminal unit and frequency-converting the upward signal inputted from the terminal unit to the in-building upward signal using the pilot signal, and outputting the in-building upward signal toward the in-building terminal line.
According to the present invention, the up-converter and the down-converter can use the pilot signal of the bi-directional CATV system to allow the converting reference signals to coincide with each other, and the down-converter can accurately restore the in-building upward signal to the original upward signal.
According to another aspect of the present invention, there is provided an in-building CATV system comprising: a reference signal generating circuit for generating a reference signal for use in frequency conversion; a down-converter disposed between a lead-in wire and an in-building transmission line for outputting a downward signal inputted via the lead-in wire toward an in-building transmission line, using the reference signal to frequency-convert an in-building upward signal inputted via the transmission line to an upward signal corresponding to a transmission frequency in a bi-directional CATV system, and outputting the upward signal toward the lead-in wire; and an up-converter disposed between the in-building transmission line and a subscriber terminal unit for outputting the downward signal transmitted on the in-building transmission line toward the subscriber terminal unit and frequency-converting the upward signal inputted from the terminal unit to the in-building upward signal using the reference signal, and outputting the in-building upward signal toward the in-building transmission line.
According to the present invention, the up-converter and the down-converter can use the reference signal generated by the reference signal generating circuit to allow the converting reference signals to coincide with each other, and the down-converter can accurately restore the in-building upward signal to the original upward signal.
According to still another aspect of the present invention, there is provided an in-building CATV system comprising: a first reference signal generating unit for generating a reference signal for use in frequency conversion; a second reference signal generating unit for generating the same reference signal as the reference signal using a pilot signal; a determining unit for determining whether or not the pilot signal is included in a downward signal; a selecting unit for selecting the reference signal generated by the first reference signal generating unit when the determining unit determines that no pilot signal is included in the downward signal, and for selecting the reference signal generated by the second reference signal generating unit when the determining unit determines that the pilot signal is included in the downward signal; a down-converter disposed between a lead-in wire and an in-building transmission line for outputting the downward signal inputted via the lead-in wire toward an in-building transmission line, using the reference signal selected by the selecting unit to frequency-convert an in-building upward signal inputted via the transmission line to an upward signal corresponding to a transmission frequency in a bi-directional CATV system, and outputting the upward signal toward the lead-in wire; and an up-converter disposed between the in-building transmission line and a subscriber terminal unit for outputting the downward signal transmitted on the in-building transmission line toward the subscriber terminal unit and frequency-converting the upward signal outputted from the terminal unit to the in-building upward signal using the reference signal selected by the selecting unit, and outputting the in-building upward signal toward the in-building transmission line.
According to the present invention, when the transmission of the pilot signal is discontinued in the bi-directional CATV system, or when such pilot signal is not transmitted by the bi-directional CATV system, the up-converter and the down-converter can allow the converting reference signals to coincide with each other, and the down-converter can accurately restore the in-building upward signal to the original upward signal.